This invention relates to liquid sampling apparatus and an associated method of sampling liquid, and more particularly to improved sampling apparatus of the type which employs a positive displacement pump for procuring the desired liquid samples. Even more particularly this invention relates to the apparatus of the type described which has improved means for sensing and controlling the quantity of the sample supplied by the pump.
Modern day concern with environmental conservation has resulted in legislation requiring careful monitoring of wastewater effluent. This task requires the use of rather sophisticated sampling apparatus for repeatedly collecting consistently accurate samples of fluid waste or industrial process fluids. (See for example, the sampling apparatus disclosed in U.S. Pat. Nos. 3,838,719, 3,927,701 and 4,022,059.)
Heretofore three basic means generally have been employed for controlling the volume or amounts of respective samples. The first could be defined as a timed pump operation in which the pump is turned on for a predetermined period of time proportionate to the volume of the sample which is desired. The weakness of this method is that the volume is affected by changes in the vertical lift of the sample, the motor speed, which may vary due to any changes in power supply, and the ambient temperature in which the motor is operating. Another weakness of this method is that the time during which the motor must operate in order to produce the desired sample is determined empirically through trial and error.
A second method often employed has utilized a sensing device which specifically counts and calculates the revolutions of the pumps associated motor shaft or armature; and assuming that a certain amount of fluid is pumped per revolution of the motor shaft, then the desired volume of the fluid can be designated in terms of the rotations of the motor shaft. Although, theoretically, this obviates any error which might result because of unexpected changes in the speed of the motor, there nevertheless is still a considerable margin of error involved when the vertical lift changes during the period of sampling, as is often the case. This method also does not compensate for variations in the lengths of the sample tubing used, or for any accidental plugging or fouling of the line which might reduce the actual volume of fluid pumped per revolution of the motor. Furthermore this system involves the use of moving parts, and is therefore rather prone to failure.
A third method of sample collection which has been utilized comprises a so-called sample collection chamber in which a vacuum is generated to draw a sample into the chamber, which is thereafter dumped. (See for example, the above-noted U.S. Pat. No. 4,022,059.) Although this vacuum system addresses the lift and accuracy question by requiring that the chamber be filled to a certain level before being dumped, nevertheless the construction and operation of this type of apparatus is very expensive and is a rather high in power consumption, which therefore reduces its utility in connection with portable sampling applications. Also, this type of vacuum system is prone to failure because of frequent leaks, which are aggravated each time the equipment is disassembled for periodic cleaning, as is required for equipment of this type.
It is an object of this invention, therefore, to provide new and improved sampling apparatus of the type described which is substantially more reliable, accurate and inexpensive to manufacture than prior such apparatus. In this connection it is an object of this invention also to provide improved means for sensing the actual quantity of fluid delivered to, or discharged from, and associated pump during each sampling operation.
Another object of this invention is to provide an improved method of sampling liquids, and which method is substantially more accurate in measuring the volume of the sample, and which is particularly suited for use with a microprocessor that controls repeated and accurate sampling operations.
A further object of this invention is to provide a novel, non-contacting, non-obstructing sensing means for sensing the presence of a liquid sample in the tubing which is associated with the inlet of the sampling pump of this apparatus.
Additional objects will be apparent from the specification, the appended claims, and the accompanying drawings.